github (actions): free disk space

Otherwise building host configurations fails with
  no space left on device

.github/workflows/ci-build.yaml

@@ -14,6 +14,8 @@ jobs:
       matrix:
         configuration: ["pulse-14", "latitude-7280", "cyberoffice"]
     steps:
+      - name: Free disk space
+        uses: jlumbroso/free-disk-space@main
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/nix-installer-action@main
       - run: nix build .#nixosConfigurations.${{ matrix.configuration }}.config.system.build.toplevel